Computer controlled printers and in particular ink-jet and piezoelectric printers have been commercially available since at least the late 1980's. Their general construction is also well known, being the subject of numerous patents world-wide. An example of this technology can be found in U.S. Pat. No. 5,455,613 entitled "Thin Film Resistor Printhead Architecture for Thermal Ink-Jet Pens" by Canfield et al. issued on Oct. 3, 1995.
In a computer controlled printer, the ink is expelled drop-by-drop in a controlled manner. In a thermal ink-jet printer a firing resistor is electrically pulsed which in turn generates a drive bubble. The drive bubble expands in the firing chamber and expels a drop of ink from the chamber. In a piezoelectric printer a piezoelectric transducer is electrically pulsed which in turn expels a drop of ink from the chamber. In both, a region of vibration in the ink in the chamber is formed by the process of expelling the drop of ink. In addition, in both, the ink in the chamber bulges out of the orifice and a generally convex meniscus across the orifice results. The meniscus is not uniformly curved; the meniscus is actually oblate and also sloshes back and forth under the influence of the vibration of the ink in the chamber. The meniscus responds to a surface tension phenomenon. The ink in the chamber and the meniscus act much like a classical mass-spring-dashpot system.
Referring to FIG. 1, reference numeral 12 generally indicates a drop 14 of ink being expelled from an orifice plate 16 on the wall of a chamber 17. Reference numeral 18 indicates the generally convex meniscus resulting after the expulsion of the drop.
Before expelling the next drop of ink, the chamber should be refilled. Refilling the chamber with ink as fast as possible is a very desirable design goal. However, if ink flows into the chamber too fast, the ink will flow out of the orifice and leak into the printer. On the other hand, refilling too slowly will cause the printer to operate unnecessarily slowly and the media throughput of the printer will be adversely affected.
In addition, before expelling the next drop from the chamber, both the vibration in the chamber must be damped out as much as possible and the meniscus flattened, or the trajectory of the next drop will be adversely affected. Specifically, if the next drop is prematurely expelled, the drop will not travel along its designed path and the quality of the resulting image will be degraded.
The effects of less than optimum damping and refilling are best shown in the graph, FIG. 2, which illustrates how the weight of the drops expelled from an ink-jet print head vary as the frequency of a firing resistor is changed. The geometry of the chamber and the chemical properties of the ink remain unchanged in FIGS. 2 and 3. The optimum firing frequency for the resistor is indicated by reference numeral 20. The chamber overshoots and is not being damped sufficiently in the area indicated by reference numeral 21.
Heretofore, to properly damp the vibration in the chamber and to achieve optimum refilling times, five hydraulic resistance variables have been optimized either through computer modeling or trial and error or both. The two parameters for ink are viscosity and surface tension, and the three geometric parameters of the print head are the length, width, and height of the ink inlet channel to the chamber.
FIG. 3 illustrates a fully damped, prior art chamber in which the problem of being under damped, i.e., overshooting, was eliminated. Reference numeral 22 indicates the optimum firing frequency for this chamber. Typically to achieve this prior damping solution, the length of the inlet channel to the chamber was lengthened and the width and the height of the channel were decreased. However, although overshooting was eliminated, the optimum firing frequency 22 was reduced as compared to the optimum firing frequency 20 in FIG. 2. The net effect was that the printer ran slower and the output of media per minute was reduced.
It will be apparent from the foregoing that although there are well known ways of dampening the vibration in the ink in printers, there is still a need for an approach that allows the printer to operate as fast as possible while tolerating the maximum hydraulic under-damping that achieves acceptable print quality.